Altermagnetism revealed by polarized neutrons in MnF ₂

Altermagnets are compensated magnets with symmetry-enforced spin split bands. Direct spectroscopic evidence for their existence has been reported in several materials. For example, inelastic neutron scattering (INS) has recently been used to witness a splitting of magnon bands in MnTe that is compatible with altermagnetism. Here, we present polarized INS results on MnF ₂ showing that it is altermagnetic. Our data reveal a splitting coming from the long-range dipolar coupling. Furthermore, using a domain-biased sample, the data reveal a nonzero chirality in the neutron scattering cross section that reverses the sign between the two magnon modes. It is this feature that clearly demonstrates altermagnetism in MnF ₂ . This finding highlights the potential for polarized INS to characterize altermagnets through its sensitivity to fine structure in magnon spectra.